Reference Number citations are a method of documenting sources using a number
in the text of the document that references a numbered bibliographic citation in
a list of works cited.

There are two basic systems for organizing Reference Number citations in a document:
alphanumeric and numbered references.

For reference number citations, repeated citations in the text are assigned
the same number.

Example: Numbered references:
For numbered references, the reference list is ordered in the
order of their appearance in the paper:

Nothing seemed so certain as the results of the early studies (1). It was
precisely this level of apparent certainty, however, which led to a number of
subsequent challenges to the techniques used to process the data (2). There
were a number of fairly obvious flaws in the data's aspect: consistencies and
regularities that seemed most irregular, upon close scrutiny (1,2).